Electrically conductive drilling fluids



United States Patent ELECTRICALLY CGNDUCTIVE DRILLING FLUIDS Paul W.Fischer, Whittier, Calif assignor to Union Oil Company of California,Los Angeles, Calif., a corporation of California No Drawing. ApplicationMay 10, 1952, Serial No. 287,250

18 Claims. (Cl. 2528.5)

This invention relates to oil-base drilling fluids, and in particularconcerns oil-base. drilling; fluids which are electrically conductive.It further relates to means for rendering nonconductive oil-basedrilling fluids electrically conductive, and to means for conductingelectric well logging operations.

In drilling oil or gas wells by means of rotary drilling tools, a hollowdrill pipe having a bit attached to its lower end is extended downwardlythrough the well bore and is rotated while the bit is pressed againstthe working face in the formation at the bottom of the hole. The actionof the rotating bit grinds away the formation as the drillingprogresses; During the drilling a fluid body known as a drilling fluidis continuously circulated down through the drill stem, through the bitand against the working face of the hole, and then back up to thesurface through the annular space between the drill stem and the wallsof the bore hole. The drilling fluid serves a number of purposes, amongwhich are coolingand lubricating the drill bit, suspending and removingcuttings from the bore hole, preventing the. flow of liquids from theformations traversed by the bore into the same by exerting a hydrostaticpressure on such formations, and fulfilling other purposes.

In locations where the underground formations traversed. and/orpenetrated by the bore. contain materials such as hydratable clays whichswell and/or disintegrate in the presence of Water, it has becomecustomary to employ drilling fluids which are relatively free of water,e. g. fluids which contain less than about 10 per cent of water, inorder to preclude or minimize the introduction of water into the bore bymeans of the. drilling fluid. Such drilling fluids are termed oil-basefluids since they almost invariably comprise a mineral oil havingdispersed or suspended therein minor proportions of various. agentsadapted to impart the requisite physical properties to the base oil.Among the most important of such agents are: weighting agents, which arehigh density inert solids adapted to increase the apparent density ofthe base. oil and thus increase the hydrostatic head provided by thedrilling fluid within the bore; wall-building agents, which arematerials such as clay or asphalt adapted to coat or plaster the wallsof the. bore with an impermeable layer which prevents escape of thedrilling fluid into permeable formations; and dispersing agents, which.serve to main tain solid components of the fluid uniformly dispersedtherein.

Among the various methods for investigating and de termining the. natureof the subsurface formations traversed. by a well bore, i. e., welllogging, those involving the measurement ofoneor moreofi the; electricalcharac: teristics. of such formations; enjoy wide. application.

Most of such methods require that the bore hole be filled with a liquidcapable of conducting relatively low voltage electric currents, and itis hence highly desirable that the drilling fluid employed in drillingthe well be adapted for such use, i. e., the drilling fluid should haverelatively good electrical conductivity. In general, however, oil-basedrilling fluids are not suflicient-ly conductive to adapt them for useduring electric logging operations, and this deficiency has greatlyrestricted their general applicability despite their other highlydesirable properties and characteristics. Typically, oil.base. drillingfluids have electrical resistivities of the order of 1X10 ohm-cms.,which is far too high to permit their use in electric logging operationswhere it is usually desirable that the bore hole fluid have aresistivity not greater than about l0 ohm-ems. and preferably belowabout 5 X 10 ohm-ems.

Inasmuch as oil-base drilling fluids normally contain a small amount ofwater, usually about 0.5-5 per cent by weight of the entire.composition, it would appear that their electrical resistivity could be.decreased simply by adding to the fluid a water-solublev electrolytewhich would dissolve in the water to form an electrically conductivephase within the body of thefl'uid. I have found, however, that theaddition of electrolytes such as sodium phosphate, sodium chloride,calcium chloride, etc. to oil-base drilling fluids has little. effect onthe electrical conductivity of the. fluid, and, furthermore, oftenadversely affects other properties of the fluid. For example, theaddition of about 2 per cent by weight of sodium phosphate to asoap-stabilized drilling fluid of the type described in U. S. Patent2,542,020 not only fails to reduce electrical resistivity appreciablybutalso increases the fluid loss value by more than fourfold. In thisrespect, the oil-base drilling fluids to which the present inventionrelates difler markedly from the so-call'ed emulsion-base fluids. Thelatter, whether they be of the oil-in-water or water-in-oil type,contain a minimum of about 10 per cent by weight of water and when theycontain about 20 or more per cent of water they can more or less readilybe rendered electrically conductive without adversely a'flecting theirfluid loss characteristics simply by dissolving an ionizable. salt inthe aqueous phase. Oil-base drilling fluids, on the other'hand, derivetheir desirable characteristics from the fact that they contain aminimum amount, e. g. less than about 10. per cent, of water and incontrast to the emulsion-base fluids, they cannot be rendered conductivesimply bythe addition of an ionizable salt or other electrolyte; It willaccordingly be understood that the term oil-base. fluid, as hereinemployed, refers to drilling fluid compositions in which the base fluidis mineral oil and in which the water content is less than about 10 percent by weight;

I am aware that certain specific oil-base drilling fluids, i. e. thosewhich contain a mixture of alkali-metal and alkaline-earth metal soapsof heat-treated rosin acids,

can be rendered conductive by incorporating in the'fluid carefullycontrolled amounts of sodium silicate. However, in order to gain thevbenefit of the use of sodium silicate in this manner, it is necessarythat the amount of alkaline-earth metal soap employed and: the watercontent of the fluid be likewise. very carefully controlled; For bestresults it is necessary to employ a. homogenizer to insure adequatedispersion of the sodium silicate: in the. base, fluid. As will. beapparent to those: skilled in. the art, such: careful control ofproportions and uncon- 3 ventional mixing procedure is very difficult toachieve under field conditions where inadequate mixing facilities renderalmost impossible the preparation of completely uniform compositions andwhere the fluid is constantly subject to contamination.

It is accordingly an object of the present invention to provide improvedoil-base drilling fluids suitable for use in electric well-loggingoperations.

Another object is to provide oil-base drilling fluids having goodelectrical conductivity.

Another object is to provide means for improving the electricalconductivity of oil-base drilling fluids, which means are adapted to useunder actual field conditions and do not adversely affect otherdesirable properties of the drilling fluid to any substantial extent.

A further object is to provide a composition of matter adapted toimparting the property of electrical conductivity to oil-base drillingfluids which are normally substantially nonconductive.

A still further object is to provide an improved method for conductingelectric Well logging operations.

Other objects will be apparent from the following detailed descriptionof the invention, and various advantages not specifically referred toherein will occur to those skilled in the art upon employment of theinvention in practice.

I have now found that the above and related objects may be realized byincorporating an electrolyte and a surface active agent into theoil-base drilling fluid. More particularly, I have found that theelectrical resistivity of I oil-base drilling fluids may be reduced tovalues within the range required for electric logging operations byadding to the fluid relatively small amounts each of a water-solubleionizable metallic compound, i. e., an electrolyte, and certain types ofsurface active agents. In

many instances the normal water content of the oil-base fluid will besuflicieut to dissolve the added electrolyte, but in other cases a smallamount of water is added for this purpose. In order to retain thedesirable general characteristics of oil-base drilling fluids, however,the water content should be maintained below about 10, preferablybetween about 4 and about 8, per cent by weight of the entirecomposition.

The water-soluble ionizable metallic compounds which are employed inaccordance with the invention to impart electrical conductivity tooil-base drilling fluids are for the most part water-soluble salts ofthe alkaliand alkaline-earth metals and alkali-metal hydroxides. Inaccordance with customary nomenclature the ammonium radical is hereinincluded within the term alkali-metal. Water-soluble salts of the heavymetals are not inoperable insofar as initially imparting electricalconductivity to the fluid is concerned, but because of the tendency ofsuch salts to hydrolyze in aqueous solution to form insolublehydroxides, their effect on electrical conductivity is not usuallypermanent. Specific examples of the foregoing class of water-solubleionizable metal compounds employed in practice of the invention includethe hydroxides of sodium, potassium and lithium, sodium chloride, bariumchloride, potassium iodide, ammonium chloride, calcium bromide,magnesium chloride, calcium nitrate, barium nitrate, magnesium nitrate,sodium sulfate, potassium sulfate, magnesium sulfate, sodium carbonate,lithium carbonate, potassium bicarbonate, sodium silicate, potassiumsilicate, sodium phosphate, ammonium phosphate, sodium polyphosphate,potassium bisulfate, sodium sulfite, sodium acetate, potassium arsenate,so dium borate, potassium dichromate, sodium cyanide, calcium nitrite,ammonium hydrogen phosphate, sodium chromate, sodium thiosulfate, sodiummolybdate potassium phosphite, sodium manganate, etc. Mixtures of suchcompounds may likewise be employed. In general, best results areobtained by employing compounds'which dissociate in water to formalkaline solutions, i. e., hydroxides and salts of strong bases and weak4 acids. The alkali-metal hydroxides, silicates, and phosphates areparticularly preferred.

The surface active agents which are employed in conjunction with theaforementioned electrolytes in accordance with the invention are of thenon-ionic type, i. e., they are compounds in which the polar andnon-polar portions of the molecule are so balanced that they aresubstantially incapable of ionization. This type of compound forms awell-recognized class of surface active agents (see Sutheim,Introduction to Emulsions, Chemical Publishing Company, 1946), and themost widely available members thereof are members of the classconsisting of (1) partial esters of polyhydric alcohols with long-chaincarboxylic acids and (2) esters of hydroxyalkyl ethers of polyhydricalcohols with long-chain carboxylic acids. The term long-chaincarboxylic acid refers to aliphatic carboxylic acids containing at least12, preferably 12 to 18, carbon atoms. The term polyhydric alcoho refersto aliphatic compounds containing at least two hydroxyl groups. A thirdimportant class of non-ionic surface active agents comprisespolyalkylene ether alcohols such as are obtained by condensing ethyleneoxide with alcohols or phenols.

The non-ionic partial esters of polyhydric alcohols with long-chaincarboxylic acids are for the most part oil-soluble hydrophobic materialswhich promote the formation of stable water-in-oil emulsions. Typicalmembers of this class of surface active agents include glycerolmono-oleate, sorbitan monoeoleate, pentaerythritol mono-oleate,propylene glycol mono-stearate, glycerol mono-ricinoleate, sorbitolmono-palmitate, the pentaerythritol mono-ester of soybean fatty acids,sorbide mono-laurate, glycerol mono-stearate, sorbitan tri-oleate,butylene glycol mono-laurate, mannitan dilauratc, etc. Many of thesurface active agents of this group are available commercially undersuch trade names as Arlacel, Emcol, Pentamul, Prostearin, Span, Tegin,etc.

The esters of hydroxyalkyl ethers of polyhydric alcohols and long-chaincarboxylic acids are for the most part hydrophilic substances which tendto promote oilin-water emulsions, particularly when the ether-alcoholgroup is of relatively high molecular weight. Examples of this class ofsurface active agents are diethylene glycol laurate, tri-ethylene glycolmono-stearate, dodecaethylene glycol mono-oleate, nonaethylene glycoldiricinoleate, diethylene glycol mono-oleate, fatty acid condensates ofethylene oxide, tetraethylene glycol mono-stearate, polyoxyethylenecondensates of sorbitan mono-oleate, polyoxyethylene condensates ofsorbitan mono-palmitate, etc. Many of such agents are availablecommercially under such trade names as Emcol DL, Emulphor AG, Glaurin,Renex, Triton, Tween, etc.

Many further examples of non-ionic surface active agents, together withtheir respective trade names and manufacturers, are listed in Industrialand Engineering Chemistry, vol. 35, pp. 107-417 and 126-130 (1943). Apreferred group of such agents comprises the hydroxypolyoxyethyleneethers of long-chain fatty acid partial esters of sorbitan which areavailable commercially under the generic trade name Tween. Such agentsare prepared from sorbitan by the following series of operations:

1. Esterification of one or more, but not all, of the hydroxyl groups ofthe sorbitan molecule with a long-chain fatty acid, e. g., oleic acid,stearic acid, palmitic acid, lauric acid, and other fatty acidscontaining at least 12 carbon atoms, to form a sorbitan partial ester.

2. Etherification of the remaining hydroxyl group or groups of saidpartial ester with ethylene oxide to form a hydroxypolyoxyethylene etherthereof. The quantity of ethylene oxide employed should be such that theether product contains from 10 to 30 oxyethylene groups.

Means for carrying out these reactions are well known in the art, andfurther details concerning the preparation of the products in questionare set forth in that portion of U. S. Patent No. 2,380,166 whichrelates to the preparation of the so-called Type B emulsifiers. Theseproducts are for the most part oily yellow liquids which are soluble inwater, acetone, alcohol and other common organic solvents. They vary inviscosity from about 250 to about 600 cps. at 25 C., depending upon thenature of the long-chain fatty acid residue, the number of such residuesin the molecule, and the length of the hydroxypolyoxyethylene chains. Asexamples of the surface active agents of this group there may bementioned hydroxypolyoxyethylene sorbitan monolaurate (Tween 20),hydroxypolyoxyethylene sorbitan monopalmitate (Tween 40),hydroxypolyoxyethylene sorbitan monooleate (Tween 80),hydroxypolyoxyethylene sorbitan trioleate, (Tween 85), etc. Furtherinformation concerning these materials and the sorbitan partial esters(Spans) is set forth in Atlas Surface Active Agents, Atlas Powder Co.,Wilmington, Delaware, 1950.

The proportions in which the water-soluble metal salt or hydroxide andthe surface active agent are incorpor- I ated in the drilling fluid areto a certain extent interdependent, i. e., with increasing amounts ofthe metal compound the amount of surface active agent may be decreased,and vice 'versa. Also, certain particular metal compounds require thepresence of more of a particular surface active agent than others, andcertain types of surface active agents are more etfective in a givenamount than-others. In general, however, the water-soluble metal salt oralkali-metal hydroxide is provided in an amount representing betweenabout 0.01 and about 5 per cent, preferably between about 0.1 and about2 per cent, by weight of the entire composition, and the surface activeagent is employed in an amount representing between about 0.1 and about8 per cent, preferably between about 0.4 and about 4 per cent, by weightof the entire composition.

As previously stated, it is necessary that the drilling fluid containsuflicient water to dissolve the Water-soluble metal salt or hydroxideso that the latter becomes dispersed in the fluid in the form of arelatively dilute solution. In many instances the drilling fluid willnormally contain suflicient water for this purpose, in which case nowater need be added along with the metal salt or hydroxide. In someinstances, however, it will be necessary to add a small amount of water.In general, it is desirable that the drilling fluid contain at leastabout 3 per cent, preferably at least about 5 per cent, by weight ofwater. The upper limit on the water content of the fluid is establishedat about per cent by weight since those fluids which contain substantialamounts of water lose the desirable characteristics of oil-base drillingfluids in general, and are more properly classifiable as emulsionbasefluids.

The use of a water-soluble metal salt or alkali-metal hydroxide incombination with a surface active agent of the above-defined class inaccordance with the invention is applicable to oil-base rilling fluidsin general, .regardless of their exact formulation. In general, alloilbase drilling fluids essentially comprise a mineral oil dispersion ofsolids and a dispersing agent which serves to maintain the solids moreor less stably dispersed in the oil. When a hydratable clay is includedas a Wall-building agent the fluid usually also contains a smallquantity of water. Almost invariably the base oil is a mineral oil, andmay be crude oil, a distillate, or a residual fraction. Very oftenblends of distillate and residual fractions are employed, e. g., a blendof a light distillate such as kerosene or Diesel fuel and a lightresidual fraction such as furnace oil or a light fuel oil. The dispersedsolids may serve solely as a weighting agent, in which case they usuallytake the form of finely-divided inert metallic com pounds such as leaddust, barytes, iron oxide, calcined clay, whiting and the like, or theymay serve as wallbuilding agents to coat or plaster the Walls of thebore with an impermeable layer which prevents escape of the drillingfluid into permeable formations traversed by the bore. Suchwall-building agent usually comprises a hydratable clay such asben-tonite, in which case a small amount of water is included in thefluid for the purpose of eflecting hydration of the clay. Asphalt isalso employed as a wall-building agent. In many instances the dispersedsolids may comprise both a weighting agent and a wall-building agent. Awide variety of materials may be employed as dispersing or suspendingagents to maintain the solids uniformly dispersed in the base oil. Forthe most part, however, the dispersing agent will comprise a metal soapof a fatty, resin or naphthenic acid. In some instances such soaps areformed in situ by incorporating into the fluid a soap-forming acid, suchas tall oil, rosin, oleic acid, sulfonic acid, linoleic acid, linseedacids, and the like and a basic inorganic compound such as sodiumhydroxide, lime, or sodium silicate. In other cases the soap may beformed in situ by incorporating an alkali-metal soap and an alkalineearth metal base into the fluid, whereby a metathesis reaction occurs toform the corresponding alkaline-earth metal soap. Soap mixtures,including mixtures of wa'ter-dispersible and oil-dispersible soaps mayalso be employed. In addition to the soap-type dispersing agents, suchmaterials as lampblack and diatomaceous earth have been employed for thesame purpose.

While the principle of the invention is applicable broadly to all typesof oil-base drilling fluids, it is particulariy applicable to thesoap-stabilized fluids of the type described in U. S. Patent 2,542,020.Such fluids are prepared by dispersing small amounts each of ahydratable clay, an alkaline-earth metal base, and an alkali metal soapof a heat-treated rosin in a suitable base oil. A partial metathesisreaction occurs between the rosin soap and the alkaline-earth metal basewhereby there is obtained a mixture of the corresponding alkalineearthmetal rosin soap and unreacted alkali-metal rosin soap. The allall-metal rosin soap employed in preparing this type of drilling fluidis obtained by reacting an alkalimetal alkali, e. g. sodium or potassiumhydroxide, with heat-treated wood or gum rosin in such a manner that thereaction is only partially complete and the saponifled product containsfrom about i to about 15 per cent of free unsaponified resin acids.

The heat-treatment of rosin, whereby the resin acids thereof areisomerized and/ or otherwise modified, is well known in the naval storesart, and may be effected in various ways to obtain modified rosinproducts which vary somewhat in their physical and chemical propertiesdependin upon the nature and extent of the heat-treatment. Thus, any ofthe various color grades of refined wood or gum rosin may be heatedunder non-oxidizing conditions at temperatures between about 250 C. andabout 350 C. for a length of time sufiicient to raise the specificrotation of the rosin from its original negative value to a valuebetween about +5 and about +15 The resulting rosin product closelyresembles the original rosin in appearance, ease of saponification,etc., but is considerably altered chemically .as evidenced by itsincreased specific rotation, increased dehydroabietic acid content,lower iodine number, etc. By carrying out the heat-treatment at somewhathigher temperatures and/or over longer periods of time, the specificrotation may be raised further, e. g., to +25 or even higher, and thedegree of olefinic unsaturation further decreased. Also, under suchconditions decarboxylation takes place with the formation ofunsaponifia'ble bodies which are usually referred to as rosin oils. Theheat-treatment of rosin to secure the desired modification of the resinacids as indicated by increase in specific rotation to a value aboveabout +5 may also be effected in the presence of catalysts at relativelylow temperatures as described in U. S. Patent 2,154,629. The catalystsemployed are of the hydrogenation type, c. g., metallic platinum orpalladium, although the treatment is carried out in the absence of addedhydrogen. The reaction which takes place is termed disproportionationsince it involves the simultaneous hydrogenation and dehydrogenation ofabietic-type acids with the consequent formation of dihydro abietic anddehydro-abietic acids and their analogues, and the resulting product isreferred to as dispropo-rtionated rosin. Similarly, the product obtainedby heat-treating rosin under conditions sufficiently drastic thatcarboxyl groups are removed from the rosin acids is termeddecarboxylated rosin, and the product obtained by heat-treating rosinunder less drastic conditions so that the change effected issubstantially only one of molecular rearrangement is referred to asisomerized rosin. All of these modified rosin products are characterizedby having been prepared by heat-treating rosin under conditions of timeand temperature, and in the presence or absence of a hydrogenationcatalyst but in the absence of added hydrogen, suflicient to raise thespecific rotation of the rosin to a value above about Any of the abovedescribed modified rosin products may be used to obtain thesaponification products employed in preparing the preferred drillingfluid compositions of the present invention. Procedure for carrying outthe saponification reaction is well known in the art, and in generalconsists merely of adding the modified rosin in the solid or moltenstate to a hot aqueous solution of the desired alkali-metal alkali andthereafter heating the mixture until the reaction is complete and theproduct contains the desired amount of water. The amount of alkaliemployed is somewhat less than that required for the completesaponification of the resin acids in order that the saponificationproduct may contain the requisite amount of free unsaponified resinacids. The concentration of the aqueous alkali is usually so adjustedthat the product obtained takes the form of a viscous liquid or thickpaste containing 60-85 per cent solids. The physical form of the productalso depends somewhat upon the type of modified rosin employed. TheSaponification product obtained from decarboxylated wood rosincontaining a substantial amount of rosin oils, for example, is arelatively fluid liquid even though it may contain only 5-10 per cent ofwater.

While any of the alkali-metal alkali saponification products of rosinwhich has been heat-treated to raise its specific rotation to a valueabove about +5 may be employed in preparing the oil-base drillingfluids, I have found that superior results, particularly with respect tothe fluid loss value of the drilling fluid are attained by employingeither of two specific products of this type. The first of suchpreferred saponification products is an alkalirnetal alkalisaponification product of resin which has been heat-treated attemperatures between about 250 C. and about 350 C. in the absence of acatalyst to such an extent that it contains only about 50-6O per cent offree resin acids, -40 per cent of unsaponifiable oils, and small amountsof phenolic materials, water, and products of unknown constitution. Aparticularly preferred product of this type is the potassium hydroxidesapenification product of such heat-treated rosin containing about 55per cent potassium resin acid soaps, about 3035 per cent unsaponifiablematerials, about 510 per cent free resin acids, and about 510 per centwater. The second of the preferred class of saponifica tion products isthe product obtained by heating rosin at a temperature of about 225 300C. for about 15-60 minutes in contact with a hydrogenation catalyst butin the absence of added hydrogen, distilling the resulting product andcollecting a fraction distilling at about 210275 C. under about 5-10 mm.pressure, and thereafter saponifying such fraction with aqueous sodiumhydroxide in the known manner. Such product is available commerciallyunder the trade name Dresinate 731. Mixtures of these two types ofsaponiried heat-treated rosin products may also be employed.

The proportions in which the components of this type of drilling fluidare employed may be varied between certain limits depending on theidentity of such components and the specific properties desired in thecomposition. Ordinarily, however, the Saponified heat-treated rosinproduct is employed in an amount representing between about 1 and about10, preferably between about 4 and about 8, per cent by weight of theentire compo sition. The alkaline-earth metal base, which is preferablycalcium hydroxide or calcium oxide, is employed in an amountcorresponding approximately to that chemically equivalent to thesaponified rosin product. When the latter is one of the preferredproducts hereinbefore described and the alkaline-earth metal base iscalcium oxide or hydroxide, the saponification product is provided inthe above-mentioned amount and the base is employed in an amountrepresenting between about 0.1 and about 5, preferably between about 0.4and about 2, per cent by weight of the entire composition. Thehydratable clay is employed in an amount representing between about 0.1and about 5, preferably between about 0.4 and about 1.2, per cent byweight of the entire composition, and the water is provided in an amountrepresenting between about 0.2 and about 10, preferably between about 1and about 5, per cent by weight of the entire composition. Theseproportions of water include any water which may be contained in thesaponified rosin product and/ or other components, and accordingly theamount of water actually added during preparation of the compositionwill be adjusted according to the water content of the other componentsso that the final composition will contain water in the above-mentionedproportions.

In determining the electrical resistivity of the drilling fluidsprovided by the invention, an electrode assembly comprising two l-inchsquare nickel plates spaced about one inch apart is immersed in a sampleof the fluid being tested, and the voltage which must be applied acrossthe electrodes to obtain a predetermined current flow through the fluid(usually 400 milliamperes) is ascertained. By calibrating the electrodeassembly against a liquid of known resistivity, the resistivity of thefluid sample being tested may be determined from such voltage reading.It has been found, however, that many drilling fluids undergo adielectric breakdown during such testing procedure. Thus, when the fluidis first subjected to the test the voltage across the electrodes may beincreased to a relatively high value, e. g., volts, before substantialcurrent flow. As soon as the current starts to flow, however, thevoltage may be substantially reduced without the flow of current fallingbelow the aforesaid predetermined value. Accordingly, in making theresistivity determination, the electrode assembly is immersed in thefluid and the voltage applied to the electrode plates is graduallyraised until the predetermined flow of current is obtained. A so-calledinitial resistivity value is determined from the applied voltage. Thevoltage is then gradually reduced, and the minimum voltage required tomaintain the predetermined current fiow is ascertained, and a so-calledultimate resistivity value is determined from such minimum voltagereading. The dielectric breakdown of the fluid is more or less permanentand the ultimate resistivity value represents the resistivity which thefluid will have during electric logging operations.

The following examples will illustrate a number of ways in which theprinciple of the invention has been The diesel fuel was a light domesticdiesel oil having an API gravity of about 31", a viscosity of about 40SUS at 100 F. and a boiling range of about 400-720 F. The saponifieddecarboxylated rosin contained 45-55 per cent of potassium resin acidsoaps, 30-55 per cent of unsaponifiable rosin oils, -10 per cent of freeresin acids and 5-10 per cent of water. The saponified disproportionatedrosin was the hereinabove described Dresinate 731.

This concentrate composition was then partially diluted with a furtherquantity of the diesel fuel in a ratio of 3 gallons of diesel fuel per 2gallons of concentrate, and the partially diluted concentrate was thenfurther diluted with fuel oil in a ratio of 17 gallons of fuel oil per 5gallons of the partially diluted concentrate. Approximately 740 parts byweight of calcium oxide were then stirred into the completely dilutedconcentrate. The fuel oil was a light domestic fuel oil having an APIgravity of about 14.5 a viscosity of about 36 SSF at 127 F. and a flashpoint of 170 F.

The finished drilling fluid had the following approximate composition:

Percent by weight Domestic diesel fuel 15.7 Light domestic fuel oil 77.7Saponified decarboxylated rosin 1.9 Saponifi cd disproportionated rosin1.9 Calcium oxide 0.9 Water 0.8 Bentonite 1.1

Table II Resistivity, Exm Ohm-cms.X1O

N Surface Active Agent Initial Ultimate 12. Alkylated aryl polyetheralcohol 1 34 5. 2 13 Hydroxypolyoxyethylene sorbitan 120 50 mono-oleate.14 Sorbitan mono-cleats 1 44 12 15 Glycerol mono-oleate 120 19 16.Hydroxypolyoxyethylene sorbitan 120 50 mono-laurate. 17 Diethyleneglycol mono-stearate 110 33 18 Propylene glycol mono-oleate 95 18 19.sorbitan tri-oleate 5 .i 120 42 1 "Triton X-100." 4 Tween 20. Tween 80.1 Span 85. 3 Span 80."

EXAMPLE III Approximately 90 parts of tall oil, 45 parts of sodiumsilicate (N grade)v 150 parts of air-blown asphalt and 45 parts of 50%aqueous sodium hydroxide were stirred into about 500 parts of lightdiesel fuel, and the resulting composition was diluted with about 540parts of light domestic fuel oil. The resulting oil-base drilling fluidwas stirred for one hour, after which it was divided in 1500 partportions. To each portion there was then added 100 parts of water andparts of hydroxypoly oxyethylene sorbitan mono-oleate (Tween 80).Approximately 10 parts of the salt indicated in the following Table IIIwere then added to each of the portions of the drilling fluid, and theresistivities of the resulting compositions were determined.

Table III Resistivity, Ohxn-cms. l0 Form. No. Salt Initial Ultimate None1, 000 1,000 Sodium bicarbonate 85 15 Versene 1 72 7. 2 'lrisodiumphosphate i8 'lrisodium polyphosphat 11 Sodium silicate 78 22 1 A sodiumethylene diamine tetra-acetate.

Table I Electrolyte W t Surface Active Agent 32 1 9. er, Expt. No.percent 1 Identity Percent 1 Identity Percent 1 Initial Ultimate None0.0 0.0 NO 0.0 1,000 1,000 0 0.0 6.7 Triton X-100 2. 4 1, 000 l, 000Trisodium phosphate 0. 6 6. 2 None 0. 0 1, 000 500 d0 0.6 6.0 TritonX100 2. 4 34 1.2 do 0.9 6.0 d 2.4 23 2.2 Sodium Phlm'iflo O. 9 7. 4 2. 3150 35 d 0. 9 7. 4 3. 4 5S 5- 2 0. 9 7. 4 2. 3 45 8.8 Sodium carbonate0. 9 7. 4 2. 3 73 24 Trisodium polyphosphate 0. 9 7. 4 2. 3 62 22 Sodiumsilicate (N-Grade). 0. 9 7. 4 2. 3 150 3 8 1 Based on weight of entirecomposition EXAMPLE II The data presented in the following Table II,illustrate EXAMPLE IV the use of a number of different emulsifyingagents of the present class in accordance with the principle of theinvention. In each experiment, approximately 100 parts of water, 10parts of trisodium phosphate and 40 parts of the indicated emulsifyingagent were added to 1500 parts of the oil-base drilling fluid preparedin Example I. The resulting fluid had the following approximatecomposition: Percent by weight Oil-base drilling fluid 90.2 Trisodiumphosphate 0.6 Water 6.8 Emulsifying agent 2.4

Approximately 400 parts of a commercial drilling fluid concentratecomprising asphalt and lime were stirred into 980 parts of light dieselfuel.

was divided into 1500 part portions, and to each portion The resultingdrilling fluid was added parts of water, 40 parts of alkylated arylpolyether alcohol (Triton X-100), and 10 parts of the salt indicated inthe following Table IV. The resistivity of each sample is indicated inthe table:

The foregoing examples illustrate electrolytes and surface active agentsof the present class for imparting electrical conductivity to severaldifferent types of freshly-prepared oil-base drilling fluids. In manyinstances, however, it is desirable to treat drilling fluids which havepreviously been used in the field. Such field fluids are almostinvariably contaminated with calcium and/ or sodium compounds, and whilethe principle of the present invention is broadly applicable to suchdrilling fluids, I have found that superior results are attained whenthe electrolyte comprises a mixture of an alkali-metal hydroxide and awater-soluble salt of a strong base and a Weak acid, e. g., sodiumsilicate, sodium carbonate, sodium phosphate, etc. Such mixedelectrolyte and the surface active agent may be added separately to thedrilling fluid, but a more convenient procedure consists in preparing apre-mixed composition which may be packaged and sold as such as aconductivity additive to be admixed with the drilling fluid wheneverdesired. Such composition suitably comprises 100 parts by weight ofwater, between about 10 and about 40 parts by weight of a mixture of analkali-metal hydroxide and a watersoluble salt of a strong base and aweak acid, and between about 20 and about 80 parts by weight of one ofthe surface active agents of the present class. If desired, awater-miscible organic solvent such as isopropyl alcohol may be includedto promote solubility of the surface active agent in the water. Theelectrolyte mixture will comprise from about 10 to about 90 per cent byweight of the alkali-metal hydroxide and from about 90 to about 10 percent by weight of the water-soluble salt. In employing such aconductivity additive composition to impart electrical conductivity toan oil-base drilling fluid, the composition is simply stirred into thefluid in an amount equal to between about and about 20 per cent byweight of the entire composition. The use of such conductivity additivecomposition is of course not limited to contaminated field fluids but isalso applicable to drilling fluids which have not previously been used.

The following example illustrates the use of a mixed electrolyte inconjunction with a typical field fluid as above described: XAMPLE V ingamounts of electrolyte as indicated in the following Table V:

the use of various Table V Resistivity, Ohm-cms. l0 Expt. No.Electrolyte Initial Ultimate None 500 75 Trisodium phosphate14 parts.120 45 {TIlSOdllJm phosphate-l4 parts 28 2 1 Sodium hydroxide-i0 parts.Trlsodium phosphate-14 parts Sodium hydroxidel0 parts. 3.2 1. 0 Sodiumsilieate14 parts"-.. {Sodium hydroxideparts. it 2 2 1 0 Sodiumsilicate-14 parts 12 The following examples are illustrative of typicalconductivity additive compositions which may be employed as describedabove:

EXAMPLE VI Parts Glycerol mono-oleate 27 Sodium hydroxide 3 Sodiumsilicate 10 Trisodium phosphate 10 Water 50 Isopropyl alcohol 10 Thiscomposition is added to the drilling fluid in an amount representingabout 10 per cent by weight of the As will be apparent to those skilledin the art, many variations in the composition of the drilling fluidand/or the conductivity additives may be made without departing from thescope of the invention. The essence of the invention lies in adding to anormally non-conductive oilbase drilling mud between about 0.01 andabout 5 per cent of a Water-soluble metal salt or hydroxide, betweenabout 0.1 and about 0.8 per cent of a non-ionic surface active agent andsufiicient water to adjust the total water content of the composition tobetween about 3 and about 10 per cent, all of said proportions beingbased on the weight of the entire composition.

The conductive drilling fluids provided by the invention may be employedin any of the various well logging methods which require that one ormore electrodes be positioned within a well bore filled with aconductive fluid. Certain of such methods comprise a determination ofthe electrical resistivity of the earth formations traversed by thebore. Others comprise measuring the so-called self-potential of suchformation. Regardless of the exact nature of the logging method,however, the herein described drilling fluids are Well adapted to use asthe conductive fluid with which the bore is filled and within which anelectrode is submerged.

Other modes of applying the principle of my invention may be employedinstead of those explained, change, being made as regards the methods ormaterials employed provided the compositions stated by any of thefollowing claims, or the equivalent of such stated compositions, beobtained.

1, therefore, particularly point out and distinctly claim as myinvention:

1. An oil-base drilling fluid comprising a minerai oil carryingsuspended solids and suflicient of a dispersing agent to maintain saidsolids dispersed in said oil, between about 3 and about 10 per cent byweight of water, between about 0.01 and about 5 per cent by weight of anelectrolyte selected from the class consisting of watersoluble metalsalts and alkali-metal hydroxides, and between about 0.1 and about 8 percent by weight of a nonionic surface active agent.

2. An oil-base drilling fluid according to claim 1 wherein thedispersing agent is a metal soap.

3. An oil-base drilling fluid according to claim 1 wherein thedispersing agent comprises a mixture of waterdispersible andoil-dispersible resin soaps.

4. An oil-base drilling fluid according to claim 1 wherein the surfaceactive agent is selected from the class consisting of (1) partial estersof polyhydric alcohols with long-chain carboxylic acids, (2) esters ofhydroxy alkyl ethers of polyhydn'c alcohols with long-chain carboxylicacids, and (3) polyalkylene ether alcohols.

5. An oil-base drilling fluid comprising a mineral oil having suspendedtherein a solid wall-building agent and a finely divided inert weightingagent and suflicient of a dispersing agent to maintain said solidsdispersed in said oil, between about 3 and about 10 per cent by weightof water, between about 0.01 and about 5 per cent by weight of anelectrolyte selected from the class consisting of water-soluble metalsalts and alkali-metal hydroxides, and between about 0.1 and about 8 percent by weight of a non-ionic surface active agent selected from theclass consisting of (1) partial esters of polyhydric alcohols withlong-chain carboxylic acids, (2) esters of hydroxy alkyl ethers ofpolyhydric alcohols with long-chain carboxylic acids, and (3)polyalkylene ether alcohols.

6. An oil-base drilling fluid according to claim 5 wherein saidwall-building agent is a hydratable clay.

7. An oil-base drilling fluid according to claim 5 wherein thedispersing agent is a metal soap.

8. An oil-base drilling fluid according to claim 5 wherein the surfaceactive agent is a hydroxypolyoxyethylene ether of a long-chain fattyacid partial ester of sorbitan.

9. An oil-base drilling fluid according to claim 5 Wherein theelectrolyte is selected from the class consisting of alkali-metalhydroxides, alkali-metal silicates, and alkali-metal phosphates.

10. An electrically conductive oil-base drilling fluid comprising (1) anormally non-conductive drilling fluid prepared by dispersing in amineral oil between about 1 and about 10 per cent by weight of analkali-metal alkali saponification product of rosin which has beenheattreated at a temperature between about 250 and about 350 C. for aperiod of time suflicient to raise its specific rotation to a valueabove about +5 said saponified rosin product containing between about 1and about 15 per cent by weight of free resin acids; between about 0.1and about 5 per cent by weight of a hydratable clay; betweenfabout 0.2and about 10 per cent by weight of water;.and an amount of analkaline-earth metal base corresponding approximately to that chemicallyequivalent to said saponified rosin product; (2) between about 0.01 andabout'5 per cent, based on the weight of the entire composition, of anelectrolyte selected from the class consisting of water-soluble metalsalts and alkali-metal hydroxides; (3) between about 0.1 and about 8 percent, based on the weight of the entire composition, of a non-ionicsurface active agent selected from the class consisting of (a) partialesters of polyhydric alcohols with long-chain carboxylic acids, (b)esters of hydroxy alkyl ethers of polyhydric alcohols with long-chaincarboxylic acids, and (c) polyalkylene ether alcohols; and (4)suificient Water to adjust the water content of the entire compositionto between about 3 and about 10 per cent by weight.

11. An electrically conductive oil-base drilling fluid according toclaim 10 wherein the electrolyte is provided in an amount representingbetween about 0.1 and about 2 per cent by weight of the entirecomposition, the emulsifying agent is provided in an amount representingbetween about 0.4 and about 4 per cent by weight of the entirecomposition, and suflicient water is provided to adjust the watercontent of the entire composition to between about 5 and about 10 percent by weight.

12. An electrically conductive oil-base drilling fluid 14 according toclaim 10 wherein the surface active agent is a hydroxypolyoxyethyleneether of a long-chain fatty acid partial ester of sorbitan.

13. An electrically conductive oil-base drilling fluid according toclaim 10 wherein the alkaline-earth metal base component of the normallynon-conductive fluid is selected from the class consisting of calciumhydroxide and calcium oxide.

14. An electricallly conductive oil-base drilling fluid according toclaim 10 wherein the alkali-metal alkali saponiiication product is thepotassium hydroxide saponification product of rosin which has beenheated at a temperature between about 200 and about 300 C. for a periodof time suflicient to raise its specific rotation to a value above about+5", and comprises between about 45 and 55 per cent of potassium resinsoaps, between about 30 and about 35 per cent of unsaponifiablematerials, between about 5 and about 10 per cent of free resin acids,and between about 5 and about 10 per cent of water.

15. An electrically conductive oil-base drilling fluid according toclaim 10 wherein the alkali-metal alkali saponification product is thatobtained by heating rosin at a temperature between about 225 and about300 C. for from about 15 to about minutes in the presence of ahydrogenation catalyst but in the absence of added hydrogen, distillingthe resulting product and collecting a fraction distilling between about210 and about 275 C. under 5-10 mm. pressure, and thereafter saponifyingsuch fraction with aqueous sodium hydroxide.

16. In a well logging method wherein at least one electrode ispositioned within a well bore filled with an electrically conductivefluid, the improvement which consists in employing as said fluid theconductive oil-base drilling fluid defined by claim 1.

17. In a well logging method wherein at least one electrode ispositioned within a well bore filled with an electrically conductivefluid, the improvement which consists in employing as such fluid theconductive oil-base drilling fluid defined by claim 5.

18. In a well logging method wherein at least one electrode ispositioned within a well bore filled with an electrically conductivefluid, the improvement which consists in employing as such fluid theconductive oil-base drilling fluid definedby claim 10.

References Cited in the file of this patent UNITED STATES PATENTS2,542,020 Fischer Feb. 20, 1951 2,573,960 Fischer Nov. 6, 1951 2,573,961Fischer Nov. 6, 1951 2,582,323 Fischer Jan. 15, 1952 2,603,605 PollokJuly 15, 1952 2,612,471 Fischer Sept. 30, 1952 2,618,604 Schaefler Nov.18, 1952 2,626,305 Krueger Jan. 20, 1953 2,661,334 Lummus Dec. 1, 1953FOREIGN PATENTS 467,562 Canada Aug. 22, 1950 803,828 France Mar. 27,1936 OTHER REFERENCES Bennett: The Chemical Formulary, vol. VI, page490, pub. 1943 by Chemical Pub. Co. of Brooklyn, New York.

Atlas Surface Active Agents, pages 59 and 60, pamphlet pub. Oct. 1948 byAtlas Powder Co. of Wilmington, Delaware.

1. AN OIL-BASE DRILLING FLUID COMPRISING A MINERAL OIL CARRYINGSUSPENDED SOLIDS AND SUFFICIENT OF A DISPERSING AGENT TO MAINTAIN SAIDSOLIDS DISPERSED IN SAID OIL, BETWEEN ABOUT 3 AND ABOUT 10 PER CENT BYWEIGHT OF WATER, BETWEEN ABOUT 0.01 AND ABOUT 5 PER CENT BY WEIGHT OF ANELECTROLYTE SELECTED FROM THE CLASS CONSISTING OF WATERSOLUBLE METALSALTS AND ALKALI-METAL HYDROXIDES, AND BETWEEN ABOUT 0.1 AND ABOUT 8 PERCENT BY WEIGHT OF A NONIONIC SURFACE ACTIVE AGENT.